This invention is concerned primarily with mitigating warpage of x-ray targets or anodes in rotary anode x-ray tubes.
Conventional targets for rotating anode x-ray tubes are essentially discs which on their front face have an annular region, constituting a focal spot track, beveled rearwardly and concentric with the axis of rotation. Usually the rear surface of the targets is concave and the cross sectional thickness is substantially uniform. The configuration has been considered a reasonably good compromise of several conflicting design objectives. For instance, the rear surface concavity economizes material and reduces the mass and, hence, the moment of inertia of the target so it may be accelerated rapidly to maximum rotational speed. But this reduction in material and the distribution of the material results in substantially reduced thermal capacity and substantially higher internal stress which sometimes cause the target to fracture and frequently causes it to undergo permanent warpage after an undesirably small number of thermal cycles. Warpage is manifested by a change in the focal track surface angle which results in part of the x-ray beam being cut off so it will not properly cover the film or other x-ray image recording medium. Targets which are thinner than customary would have insufficient thermal capacity for most x-ray diagnostic procedures.
As is well known, early x-ray tube targets were made of essentially pure tungsten. Eventually x-ray technics were adopted that called for high electron beam currents and voltages and high duty cycles which produced rapid deterioration of the focal spot track because of an inability of the target to conduct the intense heat away rapidly enough from the focal spot. The general remedy was to increase rotational speed of the targets up to as high as 10,000 rpm. When a radiologist is fluoroscoping, the x-ray tube is operated at low power and low target rotational speed. When something of interest is observed for which a radiograph is desired, the target must be stepped up to maximum rotational speed in the least possible time for a high power exposure and, hence, it is desirable for the target to have low inertia.
One way to reduce inertia is by making the body of the target of molybdenum which has lower density than tungsten but does not produce x-radiation as efficiently as tungsten. Hence, it became the common practice to overlay the focal spot track with a higher density alloy of tungsten and up to about 10% of rhenium which alloy has known desirable properties. In some cases as much as 35% of scarce and expensive rhenium is used. Moreover, the bimetal action between the tungsten-rhenium surface layer and the molybdenum substrate is believed to be a major factor in anode target warpage. Molybdenum has a coefficient of expansion about 12% greater than tungsten and therefore causes internal stresses to be developed that then to straighten out the target, that is, the target warps such that the beveled targed focal track surface becomes curved and deflects forwardly toward the electron beam source. Another major factor contributing to warpage is the circumferential or hoop stress developed at higher temperatures and speeds in targets having conventional shape.
Attempts have been made to reduce target warpage by reducing bimetal action. One attempt involved including in the molybdenum substrate as much as 5% tungsten. Another attempt involved adding a tungsten interlayer inside of the molybdenum substrate beneath the tungsten-rhenium layer near the rear surface of the target to counteract the bimetal action of the tungsten-rhenium surface layer and the substrate due to their differences in thermal expansion. These approaches have a number of disadvantages: (1) the addition of the tungsten layer is expensive and difficult to make uniform, (2) any nonuniformity of the layer causes serious balancing problems, (3) the more brittle nature of tungsten makes the target more prone to fracture, and (4) the addition of the tungsten, particularly at the outer diameter of the target, increases the moment of inertia of the target markedly and thereby increases the time for the rotating anode to accelerate to high speed such as 10,000 rpm.